1. Field of the Invention
The present invention relates to a composition used for encapsulating an optical semiconductor element, and more specifically, relates to a resin composition for encapsulating an optical semiconductor element that contains an epoxy-modified branched silicone resin and yields a cured product which exhibits excellent levels of hardness and crack resistance.
2. Description of the Prior Art
Conventionally, epoxy resin compositions have been widely used for encapsulating optical semiconductor elements. These epoxy resin compositions typically comprise an alicyclic epoxy resin, a curing agent and a curing catalyst, and a casting or transfer molding method or the like is typically used to encapsulate the optical semiconductor element, by pouring the epoxy resin composition into a mold inside which the optical semiconductor element has been positioned, and then curing the resin composition.
However in recent years, as the brightness and power output of LEDs have increased, discoloration and degradation problems have started to emerge for epoxy resins. In particular, transparent epoxy resins tend to suffer from yellowing upon exposure to blue light or ultraviolet light, resulting in a shortening of lifetime of the LED element.
As a result, silicone resins, which exhibit excellent heat resistance and light resistance, are now starting to be used, although the strength of the cured resin is weaker than an epoxy resin. Accordingly, high-hardness rubber-like silicone resins have been proposed for use in encapsulation applications (see Patent Document 1 and Patent Document 2).
However, these high-hardness silicone resins have poor adhesiveness, and in encased light emitting semiconductor devices, namely devices in which a light emitting element is placed inside a ceramic and/or plastic casing, and the inside of the casing is then filled with the silicone resin, thermal shock testing at −40 to 120° C. tends to cause the silicone resin to detach from the ceramic or plastic of the casing.
In order to improve the resistance to thermal shock, silicone resins containing epoxy groups have been proposed (see Patent Document 3). However, these silicone resins are synthesized by condensing an epoxy group-containing silane and a silanol, and the cured products of these silicone resins tends to be brittle and have a low elastic modulus. As a result, an LED encapsulated with this type of resin tends to develop cracks within the resin during temperature cycle testing.
Known materials for resolving this cracking problem include compositions comprising an epoxy resin and a silsesquioxane containing at least two epoxy rings (see Patent Document 4), and compositions comprising an epoxy resin and a silicone resin containing an isocyanuric acid derivative group (see Patent Document 5). However, even these compositions cannot be claimed to yield cured products that offer completely satisfactory resistance to cracking during temperature cycle testing.
[Patent Document 1] US 2002/0145152 A1
[Patent Document 2] US 2002/0190262 A1
[Patent Document 3] U.S. Pat. No. 5,492,981
[Patent Document 4] JP 2005-263869 A
[Patent Document 5] JP 2004-99751 A